Evaluation of fire performance of the fuel system is limited to the fire testing of the hydrogen and compressed natural gas (CNG) fuel tanks and thermally-activated pressure relief device “TPRD” itself. According to a NHTSA report, between 2000 and 2008, there have been over 20 failures of CNG tanks onboard vehicles. The single largest cause of these failures (over 50%) was fire. These CNG cylinder failures have occurred on OEM passenger vehicles, as well as OEM transit buses. Note that the effect of localized fires is more pronounced on cylinders of longer length, as TPRD locations are typically spaced far apart. Filament wound or composite overwrapped pressure vessels (COPVs) are highly susceptible (epoxy in the composite shell degraded performance at higher temperatures) to rapid degradation due to localized fires. Rapid-activating Pressure Relief Devices (PRDs) are critical to the protection of all such fuel tanks, but historical tank failure data indicates that these devices are not optimally designed to mitigate the effect of localized fires.
The fire protection of hydrogen and CNG tanks for vehicles currently relies on the use of TPRDs. Most relevant standards (e.g., NGV2) specify that this thermally-activated pressure relief device shall only activate when exposed to heat. PRDs are not used as the excessive pressure required for activation will not be achieved if the tank is only partially filled.
Typically the TPRDs can be found attached to a valve at one or both ends of a tank. As specified in the bonfire test requirements of various CNG and hydrogen tank standards, the length of the fire must be set to 1.65 m to evaluate the effectiveness of a TPRD to protect a tank. Note that current TPRD designs will only function if they are exposed to excessive heat, thus a fire occurring on a tank, remote from TPRD, will not activate that device. Thus on tanks exceeding this length (1.65 m), it is often necessary to introduce high pressure piping and additional TPRDs along the tank to ensure at least one TPRD is within the bonfire. This usually results in added cost, weight, and still no reliable three-dimensional fire protection system.